Material emitting only in dark parts of an object
Yes, this can be achieved with a Shader to RGB node. This lets you have a diffuse shader to compute which parts are lit, which you can then pass into an emission shader to make it glow.
Nodes:

Result:

I use a color ramp to separate between the lit and unlit areas, and then feed that into a mix shader so the lit parts are shaded with the emission, and the unlit with the diffuse shader.
If you want to achieve a smoother gradient, you can pass the value of the color ramp into the strength of the emission, so that the more lit parts also emit more.